Date:   Sun, 29 Jan 2023 21:18:24 -0500
From:   Alan Stern <stern@rowland.harvard.edu>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Boqun Feng <boqun.feng@gmail.com>,
        Andrea Parri <parri.andrea@gmail.com>,
        Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>,
        will@kernel.org, peterz@infradead.org, npiggin@gmail.com,
        dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
        akiyks@gmail.com, dlustig@nvidia.com, joel@joelfernandes.org,
        urezki@gmail.com, quic_neeraju@quicinc.com, frederic@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/2] tools/memory-model: Make ppo a subrelation of po
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On Sun, Jan 29, 2023 at 03:09:00PM -0800, Paul E. McKenney wrote:
> On Sun, Jan 29, 2023 at 01:43:53PM -0800, Boqun Feng wrote:
> > in lock.cat: 
> > 
> > 	(* Allow up to one unmatched LKW per location; more must deadlock *)
> > 	let UNMATCHED-LKW = LKW \ domain(critical)
> > 	empty ([UNMATCHED-LKW] ; loc ; [UNMATCHED-LKW]) \ id as unmatched-locks
> > 
> > we rule out deadlocks from the execution candidates we care about.
> 
> Thank you, Boqun!

Actually that's only part of it.  The other part is rather obscure:

(* Generate all co relations, including LKW events but not UL *)
let co0 = co0 | ([IW] ; loc ; [LKW]) |
	(([LKW] ; loc ; [UNMATCHED-LKW]) \ [UNMATCHED-LKW])

Implicitly this says that any lock with no corresponding unlock must 
come last in the coherence order, which implies the unmatched-locks rule 
(since only one lock event can be last).  By itself, the unmatched-locks 
rule would not prevent P3 from executing before P1, provided P1 executes 
both its lock and unlock.

Alan